This application claims foreign priority under 35 USC § 119 to Korean Patent Application No. 10-2003-0041104 filed on Jun. 24, 2003, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus and a method for enhancing a transfer rate in a wireless local area network (LAN) using a distributed coordination function (DCF). More particularly, the present invention relates to an apparatus and a method for presenting a novel format of a media access control (MAC) frame that enables the use of a direct link protocol (DLP) and multiple channels, thereby reducing contention among stations, and enhancing a transfer rate by using the MAC frame format in infrastructure-based wireless communications.
2. Description of the Related Art
As networks are increasingly being constructed in a wireless manner, and requests for the transfer of large volume of multimedia data continue to rise, there is a need for studies on methods for effective transfer in wireless LANs. There are two methods for improving the performance of wireless LANs with regard to the transfer of various multimedia data. The first method is a method of securing the quality of service (QoS) in a MAC level in order to effectively transmit data within a given period of time over conventional wireless LAN schemes in which a plurality of stations share a single channel. At present, an IEEE 802.11e group makes an effort to unify standards for improving QoS. The second method is a method of increasing bandwidth by allowing stations to acquire physical channels using multiple channels rather than a single channel in a basic service set (BSS).
A conventional transfer method in a wireless LAN employs a carrier sense multiple access with collision avoidance (CSMA/CA) protocol in which a plurality of stations share a single channel to perform transmission. If a BSS is established after the scan and join processes, the stations check whether they are participating in the same BBS when communications are made among them, using service set identifiers (SSIDs) and basic service set identifiers (BSSIDs). Before transmitting frames, the stations check whether a channel is idle by means of two methods depending on CSMA/CA. A method of checking channel conditions determines whether a channel is busy or idle by checking a received signal strength indication (RSSI) value of the channel that is physically used. Furthermore, there is a method of sensing a virtual carrier which determines the time when the stations use the channel by using a network allocation vector (NAV). In such a CSMA/CA contention method, since a single channel is shared, channel conditions are determined using physical carrier sensing (CCA indication) or virtual carrier sensing (NAV) so as to avoid collisions.
As shown in FIG. 1, a sending station STA1 210 sends a RTS (Request To Send) frame 211 to a receiving station STA2 220 present in the same BSS before transferring data 212 to STA2 220, in order to determine whether STA2 220 can receive the data 212. STA2 220 sends a CTS (clear to send) frame 221, i.e. a control frame, which notifies STA1 210 that STA2 220 can receive the data 212 and allows STA1 210 to transfer the data 212. STA1 210 then sends the data 212. In this process, NAVs are set up in the remaining stations, including STA3 230, but not including STA1 210 and STA2 220 present in the same BSS. STA3 230 considers the communication channel to be busy during NAV periods 231 and 232, thus no data is sent.
In CSMA/CA protocol wherein a single channel should be shared, there have been conducted various studies to secure transmission within a given period of time, i.e. to improve QoS, to thereby overcome limitations on bandwidth. A DLP is used when a point to point (P2P) network is required among the stations included in the BSS. DLP is a protocol that was proposed by the 802.11e standard in order to overcome problems occurring due to the sharing of a single channel and to improve QoS. Since data can be directly transmitted without passing through an access point (AP) if the DLP is used, it is possible to reduce a propagation delay and the total number of transmission times. Moreover, as there is no need for a MAC processing time in an AP, the use of the DLP has an advantage in that a greater amount of data can be transmitted within a given period of time.
The process of setting up a DLP according to the configuration of the DLP shown in FIG. 2 will be described. QSTA-1 210 that is a DLP requester sends a DLP request frame to an AP 250 (1a). At this time, the DLP request frame contains information on a data transfer rate, the capabilities of the station, and the like. Next, the AP simply forwards the DLP request frame to QSTA-2 220 that is a receiving station (1b). QSTA-2 220 confirms the DLP request frame received from the AP 250, and then transmits a DLP response frame, which contains information on whether to participate in a direct link 240, to the AP 250 (2a). The DLP response frame contains a status code for informing the results of the DLP request, a data transfer rate, the capabilities of the station, and the like. Finally, the AP 250 simply forwards the DLP response frame to QSTA-1 210 (2b). A series of such four processes is called a 4-handshake process of the DLP. For reference, the structures of the DLP request frame and the DLP response frame in the related art are shown in FIG. 3.
In conventional techniques by which a plurality of stations share a single channel, a point at issue is how the plurality of stations share the maximum transfer rate of the single channel (e.g., 54 Mbps in case of 802.11a). There have been developed many MAC algorithms in view of QoS so as to transfer data within a given period of time. DLP is one of these methods, which directly transfers data through a direct link without passing through an AP under the condition that P2P communications should be made after the DLP is set up. However, even though DLP is used, it is difficult to make use of the advantages of the direct link if contention is increased due to the presence of many stations in a BSS. Therefore, a method that enables efficient communications while making use of the advantages of DLP even when there is the presence of a plurality of the stations in a wireless LAN, is required.